Been doing my 3x20s for a few weeks now-- 2 times per week, with occasional road or mtb rides thrown in on the weekends.

Last night I was unable to get my HR up to what I was previously holding without a much higher perceived effort. I settled in and did the intervals at about 8-10 bpm below my normal average. My average speed however was the same. The constant speed/lower heart rate seems like improvement, but higher perceived effort for a lower heartrate does not. FWIW, I had a very tiring weekend (out-of-town wedding) with no riding.

Was this just a fatigue/motivation issue, or is something else going on? Perhaps my target HR is wrong?

Thanks,

soup

Make the next week an easy week. During my training, after about 3 weeks my heart rate for those intervals is lower, and I can't get near maximum heart rate, even on steep hills or during flat out sprinting.

If I take an easy week, everything is restored, and mostly to a higher spec. For an easy week, I'd not do the 2x 20s, take 2 days off the bike, and make two rides recovery rides (really low effort). For the weekend, I'd still do close to normal duration rides, but make them much more about taking it easy and having fun. Do you monitor your resting heart rate in the mornings? That can be useful help to track fatigue.

Probably just fatigue of some kind. It's not overtraining.